In order to examine the functional organization of single neurons in the central visual system and the manner in which that development proceeds in early life, quantitative electrophysiological techniques are used to examine the visual response properties of neurons in the lateral geniculate body and visual cortex of normal adult cats and of young kittens reared under various visual conditions. A laboratory computer system is used to present a set of analytical test stimuli based on those used in Fourier optics on a CRT, and to store and analyze neuronal responses. Electrical stimuli are delivered to various sites afferent and efferent to the region under study in a particular experiment. By combining a standard set of visual test stimuli with these electrical stimulation methods, the manner in which different sub-aspects of the visual information relayed from the retina are parcelled out by the LGN and processed in the striate and extrastriate corte is studied. Directly comparable information is obtained from neurons in neonatal kittens and visually deprived kittens in order to establish which aspects of visual function are susceptible to environmental influence and the kind and degree of this susceptibility.